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 9)  If more than one PG becomes inoperative during autoland, the confirmation signal from BCU 2 to the FCCs is removed. At an altitude above 200 feet, the BTBs and DCIRs return to their original position before autoland. Below an altitude of 200 feet, only the DCIRs close, maintaining isolation of the ac power. The BTBs remain in the last configuration until the autoland isolation request is removed.
Anticycyling and lockout functions are provided by the BCU for all switch inputs into the BCU. The BCU also provides lockout functions for breaker control. The anticycling function of the BCU provides debouncing of the switch input signals into the BCU. This prevents breaker operation from an intermittent signal on switch inputs to the BCU. The BCU provides a lockout function if a power transfer is in process. Any switch requests during a power transfer are locked out until the power transfer is complete. If a breaker trip signal and breaker close signal are requested at the same time, the BCU will give priority to the breaker trip signal and ignore the breaker close signal.The power transfers between generator power, external power, and auxiliary power are completed without a break in power if the conditions for paralleling are met in 3.5 |0.5 seconds after request for the power transfer. If the conditions for paralleling are not met, the power transfer is completed with a power interruption equal to or less than 50 milliseconds. The following functions allow the BCU to perform no-break power transfers: 1) APU Interface Signals
 a)  BCU No. 1 provides two speed-jogging signals to the APU. BCU No. 2 provides two speed-jogging signals to BCU No. 1. If BCU No. 1 is not requesting the APU to speed-jog, the speed-jogging signals from BCU No. 2 are sent to the APU by BCU NO. 1. These signals tell the APU to increase speed or dcrease speed to allow power transfer between external power and auxiliary power.
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MAINTENANCE MANUAL

 2) Auto-paralleling a) The BCU auto-paralleling function senses a phase of the adjacent synchronizing bus, opposite synchronizing bus, external power, and auxiliary power voltages. The BCU compares the voltage, frequency, and phase angle of the adjacent synchronizing bus with either the opposite synchronizing bus, external power, or auxiliary power to perform generator paralleling and no-break power transfers.
 3) Frequency Reference Control a) During normal operation, BCU No. 1 provides a master frequency reference for the adjacent and opposite GCUs so that generator paralleling and no-break power transfer can be performed. b) During transfers from IDG power to external or auxiliary power, the frequency reference to the GCUs is switched to that frequency of the external or auxiliary power source involved in the transfer. If the SSB is open, only the frequency reference to the adjacent GCUs of the BCU requesting the power transfer is switched. If the SSB is closed, all GCUs receive the frequency reference.
 4) Ground Power Cart Configuration a) The BCU is compatible with three different types of ground power cart circuitry connected across pins E and F of the external power plug. The three different types of ground power cart circuitry are as follows:
 1._Pin F is shorted to Pin E. The BCU supplies +28 volts dc to pin F which is returned through pin E.
 2._Pin F is isolated from pin E. Pin E is supplied +28 volts dc from the ground power cart.
 3. Pin F supplies +28 volts dc to hold a manually set
_ contactor on the ground power cart in the closed position. The closed contactor sends the +28 volts dc from pin F to pin E and connects the 3-phase power to the plug.
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